The retinal pigment epithelium (RPE) plays a critical role in the regulation of retinal and choroidal function in normal and disease states. Due to limited availability of human tissues, an in vitro cell culture system is needed; therefore, we have developed and characterized the primary cell lines of human RPE from donor eyes obtained from eye banks. Using human RPE cell cultures as an in vitro model, we examined various roles of RPE in the pathophysiology of retinal disorders. Human RPE cultures exposed to bacterial lipopolysaccharide (LPS), tumor necrosis factor alpha (TNF-`), and interleukin 1 alpha and beta (IL-1` and IL-1~) secreted large amounts of interleukin 6 (IL-6). The dose-dependent response to inflammatory mediators was rapid and sustained in the presence of stimulants. Upon withdrawal of stimulants, IL-6 levels dropped to prestimulatory levels within 12 hours, suggesting the reversible response of RPE to inflammation. IL-1` and IL-1~ are the most potent inducers of IL-6, followed by TNF-` and LPS. Growth factors did not induce IL-6 production by RPE cells. Western blot analysis of secreted IL-6 revealed multiple molecular forms, indicating posttranslational modifications that involve glycosylation, as was observed in human lymphoid cells. Human IL-6 cDNA probes were prepared to examine the regulation of gene expression. Northern blotting of total RNA prepared from RPE cultures treated with inflammatory mediators correlated with the observed patterns of IL-6 secretion. The results clearly show that human RPE cells respond to specific inflammatory signals or infections and secrete IL-6. Since IL-6 is a multipotent cytokine, it may in turn perpetuate immune reactions in the pathogenesis of retinal and choroidal diseases.